I've found receipts from PayPal, Google Play and Google Checkout, order confirmations from eBay, Amazon and other shopping sites. Gmail automatically detects receipts, just like it finds notifications, social updates, forum messages and other categories of messages.
The Smart Label feature from Gmail Labs creates filters that automatically label messages and convert system labels like ^smartlabel_receipt to user labels like Receipts. You can manually do that for receipts: click the arrow from the search box, type label:^smartlabel_receipt in the "has the words" field, click "create filter with this search", click "OK" and ignore the warning, select "Apply the label" and then "new label", create a label called "Receipts", check "Also apply filter to * matching conversations" and click "Create filter". You'll get a receipts label: